In a conventional electro-photographic type image forming apparatus, a developing roller is provided for supplying toner to a surface of a photosensitive drum where an electrostatic latent image is formed. Here, rotational speed of a developing roller can be changed between a higher speed at which toner supply to the photosensitive drum is carried out for performing developing operation and a lower speed at which the developing operation is not performed in order to prolong service life of the developing roller.
More specifically, a drive mechanism capable of changing a speed of a drive force is provided between a motor that generates drive force and the developing roller. The drive mechanism is adapted to supply decelerated drive force to the developing roller when the developing roller does not perform developing operation.
Japanese Patent Application Publication No. 2008-101644 discloses such drive mechanism capable of changing a speed of a drive force. The drive mechanism includes a speed-change mechanism in which two pairs of gears having a distance between shafts of gears equal to each other are provided. Further, deceleration ratio of the first pair of gears is different from that of the second pair of gears. The speed-change mechanism is adapted for arranging the two pairs of gears such that when the first pair of gears are meshedly engaged with each other, the second pair of gears are disengaged from each other. The drive mechanism also includes a change-over mechanism adapted to change a combination of gears to be engaged with each other by moving the speed-change mechanism in an axial direction.